Please try as follow,
Date ldt_in_date
Long ll_y , ll_m
// Input date.. I use Today() as input date..
ldt_in_date = Today()
ll_y = Year (ldt_in_date)
ll_m = Month(ldt_in_date)
IF ll_m = 1 THEN
ll_m = 12
ll_y --
ELSE
ll_m --
END IF
MessageBox( "Last Month Start date" , String( Date(ll_y, ll_m, 1) ) )
MessageBox("Last Month End date" , String( RelativeDate(Date(Year(ldt_in_date), Month(ldt_in_date), 1), -1)))
Regards,
Jahir